Today's cellular systems are plagued by dropped calls, especially when drivers enter dead spots. Generally, a “dead spot” is a geographic area of coverage in a cellular system in which for one reason or another cellular resources are insufficient to handle subscriber demand. Many of these call drops are predictable—carriers know where they don't have adequate coverage, or where coverage is interrupted by hills, tunnels, buildings, etc. But that means that the cellular operator will often know that a call is likely to drop. This is especially true with the enhanced location service that some carriers and service providers are looking at deploying.
Although current telecommunication systems are generally highly reliable, there are occasions in which an established telephone call between two devices will be prematurely dropped. In a wired telecommunication system established calls are dropped very infrequently.
However, in a wireless telecommunication system, such as a cellular telephone system, established telephone calls are dropped somewhat more often than in the wired system. There are a number of reasons why a telephone call with a wireless telephone may be dropped prematurely. One reason for dropped calls is that the wireless telephone has left the coverage area of the wireless system. As is well known, cellular telephone systems are divided into cells, each of which is served by a base station which communicates with wireless telephones (Le., mobile telephones) located within the cell. All the cells together make up the coverage area of the cellular system. If a mobile telephone travels outside the coverage area, the wireless communication channel between the mobile telephone and the wireless system will be lost and the call will be dropped. In some situations the mobile telephone may be entering the coverage area of another cellular service provider. In such a case, the call may continue only if the mobile telephone has roaming privileges in that other cellular system.
Another reason that an established call may be dropped in a cellular system is due to handoff. As is well known, when a mobile telephone travels from one cell to another cell, the call is handed off from the base station serving the one cell to the base station serving the other cell. However, in some situations the base station serving the other cell may not have any radio channels available for communication with the mobile telephone at the time of the handoff. In such a situation the established call with the mobile telephone will be dropped.
Still another reason for dropped calls in a cellular system is coverage holes, which are areas in the geographic serving area which do not receive signals from the system for some reason. Such coverage holes include both indoor and outdoor areas. If a mobile telephone enters a coverage hole where coverage is not available, radio communication with the serving base station will be lost and the call will be dropped. However, calls in a cellular system may also be dropped due to RF interference and equipment failures.
Dropped calls are very inconvenient to callers. Re-establishment of the call is left to the parties to the call and neither party knows the intention of the other party. This may result in both parties attempting to call the other party, which may result in the connection being blocked. Alternatively, both parties may assume that the other party is going to initiate the call, resulting in no re-establishment of the call. The dropped call situation is even worsened when there is no indication to either party for the reason of the dropped call.
These dropped calls often result in the parties not being able to fully communicate with each other. It also results in loss of revenue to the service providers when the cellular system is not in use due to premature termination of an established call.
Thus there is a need for an improved method and apparatus for anticipating call interruption in a telecommunication network.